A conspicuous clay ovoid in Nakhla: evidence for subsurface hydrothermal alteration on Mars with implications for astrobiology.
A conspicuous clay ovoid in Nakhla: evidence for subsurface hydrothermal alteration on Mars with implications for astrobiology.
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纳赫拉的一个明显的粘土卵形体：火星地下热液蚀变的证据对天体生物学的影响。
DOI：
10.1089/ast.2013.1069
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发表时间：
2014
期刊：
影响因子：
4.2
通讯作者：
Chatzitheodoridis E
中科院分区：
文献类型：
--
作者：
Chatzitheodoridis E
在Nakhla火星陨石中发现了一个明显的生物形态卵圆形结构，它由纳米晶富铁皂石粘土和非晶物质组成。卵形体是土生土长的Nakhla和发生在一个后期形成的无定形mesostasis区域的流纹岩组合物，是间质的两个单斜辉石颗粒与铝丰富的轮辋，并包含针状磷灰石晶体，橄榄石，硫化物，富钛磁铁矿，和一种新的矿物的蔷薇辉石组。为了推断卵形体的起源，采用了大量的分析工具，包括扫描电子显微镜和背散射电子成像、波长色散X射线分析、X射线测绘、拉曼光谱、飞行时间二次离子质谱分析、高分辨率透射电子显微镜成像和原子力显微镜地形测绘。卵形体的同心壁围绕着最初的中空体积，并表现出内部分层的对比纳米纹理，但均匀的化学成分，并可能继承其整体形状从一个预先存在的囊泡中的中稳态玻璃。最终的富Fe相纤维层覆盖卵形壁结构的内表面。有证据表明，Nakhla的母岩经历了附近火流星撞击的冲击事件，熔化了辉石的边缘和间隙物质，并启动了快速冷却流体的火成热液系统，这些流体逐渐与融化的永冻层流体混合。急剧的温度梯度是负责结晶的铝丰富的单斜辉石轮辋，蔷薇辉石，针状磷灰石，和淬火的介稳态玻璃和囊泡。在卵形结构的形成过程中，情节流体渗透事件导致周围的囊泡壁的皂石皮沉淀，改变磁黄铁矿白铁矿，然后隔离的卵形壁结构从系统的其余部分通过沉积一层铁的氧化物/氢氧化物。碳酸盐、岩盐和硫酸盐最后沉积在间隙空间和沿着裂缝中。在三个看似合理的相互竞争的假设中，这种特殊的非生物情景被认为是对纳赫拉卵形结构形成的最合理的解释，尽管缺乏令人信服的生物起源证据，但很明显，火星地下含有生命可以发展的生态位环境。关键词：生物形态-粘土-寻找生命（生物特征）-火星陨石-热液系统。Astrobiology 14，651-693.
A conspicuous biomorphic ovoid structure has been discovered in the Nakhla martian meteorite, made of nanocrystalline iron-rich saponitic clay and amorphous material. The ovoid is indigenous to Nakhla and occurs within a late-formed amorphous mesostasis region of rhyolitic composition that is interstitial to two clinopyroxene grains with Al-rich rims, and contains acicular apatite crystals, olivine, sulfides, Ti-rich magnetite, and a new mineral of the rhoenite group. To infer the origin of the ovoid, a large set of analytical tools was employed, including scanning electron microscopy and backscattered electron imaging, wavelength-dispersive X-ray analysis, X-ray mapping, Raman spectroscopy, time-of-flight secondary ion mass spectrometry analysis, high-resolution transmission electron microscope imaging, and atomic force microscope topographic mapping. The concentric wall of the ovoid surrounds an originally hollow volume and exhibits internal layering of contrasting nanotextures but uniform chemical composition, and likely inherited its overall shape from a preexisting vesicle in the mesostasis glass. A final fibrous layer of Fe-rich phases blankets the interior surfaces of the ovoid wall structure. There is evidence that the parent rock of Nakhla has undergone a shock event from a nearby bolide impact that melted the rims of pyroxene and the interstitial matter and initiated an igneous hydrothermal system of rapidly cooling fluids, which were progressively mixed with fluids from the melted permafrost. Sharp temperature gradients were responsible for the crystallization of Al-rich clinopyroxene rims, rhoenite, acicular apatites, and the quenching of the mesostasis glass and the vesicle. During the formation of the ovoid structure, episodic fluid infiltration events resulted in the precipitation of saponite rinds around the vesicle walls, altered pyrrhotite to marcasite, and then isolated the ovoid wall structure from the rest of the system by depositing a layer of iron oxides/hydroxides. Carbonates, halite, and sulfates were deposited last within interstitial spaces and along fractures. Among three plausible competing hypotheses here, this particular abiotic scenario is considered to be the most reasonable explanation for the formation of the ovoid structure in Nakhla, and although compelling evidence for a biotic origin is lacking, it is evident that the martian subsurface contains niche environments where life could develop. Key Words: Biomorph—Clays—Search for life (biosignatures)—Martian meteorites—Hydrothermal systems. Astrobiology 14, 651–693.
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